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Cyclopropenyl Cation - the Simplest Huckel’s AromaticMolecule - and its Cyclic Methyl Derivatives in Titan’s

Upper Atmosphere.Arshad Ali, Edward.C. Sittler, Dennis Chornay, Bertrand Rowe, Cristina

Puzzarini

To cite this version:Arshad Ali, Edward.C. Sittler, Dennis Chornay, Bertrand Rowe, Cristina Puzzarini. CyclopropenylCation - the Simplest Huckel’s Aromatic Molecule - and its Cyclic Methyl Derivatives in Titan’s UpperAtmosphere.. Planetary and Space Science, Elsevier, 2013, 87, pp.96-105. �10.1016/j.pss.2013.07.007�.�hal-00916307�

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Cyclopropenyl Cation - the Simplest Huckel’s Aromatic Molecule - and its Cyclic Methyl Derivatives in

Titan’s Upper Atmosphere

A.Ali a,b,*, E.C.Sittler Jr.a, D.Chornaya,b, B.R.Rowec, C.Puzzarinid*

aNASA Goddard Space Flight Center, Greenbelt, Maryland 20771,USA

bDepartment of Astronomy, University of Maryland, College Park, Maryland 20742, USA

cInstitut de Physique de Rennes, Equipe: Astrochimie Experimentale, CNRS, Universite de Rennes 1,

Campus de Beaulieu, 35042 Rennes Cedex, France

dDipartimento di Chimica “G.Ciamician” Universita di Bologna, via Selmi 2, 40126 Bologna, Italy

*Corresponding author: ashraf.ali@nasa.gov; cristina.puzzarini@unibo.it

Abstract

The recent measurements by Cassini Ion Neutral Mass Spectrometer (INMS) showed the presence of

numerous carbocations and shed light on their composition in Titan’s upper atmosphere. The present

research identifies an important class of ion-molecule reactions proceeding via carbocation collision

complexes, and its implications in the chemistry of Titan’s thermosphere and ionosphere. An analysis

(based on the kinetics and dynamics of the elementary chemical processes identified) of the Cassini

measurements reveals the mechanism of formation of the three- membered Huckel aromatic rings-

Cyclopropenyl cation and its cyclic methyl derivatives. For carbocations, a nonclassical three-carbon-

center two-electron-bond structure is no longer a controversial topic in chemistry literature. Emphasis

has been placed on a future coordinated effort of state-of-the-art laboratory experiments, quantum-

chemical calculations, and astronomical ALMA and JWST observations including planetary in situ

measurements at millimeter and submillimeter wavelengths to elucidate the structure, energetics and

dynamics of the compositions of carbocations detected by Cassini cationic mass spectrometry. The

cabocation chemistry in Titan’s upper atmosphere has a possible bearing on the organic chemistry and

aromaticity in the atmosphere of primitive earth.

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Introduction:

Titan is the largest moon of Saturn and the second largest moon in our solar system. The recent

measurements by instruments (Niemann et al., 2002; Waite et al., 2004; Young et al., 2004) onboard the

Cassini spacecraft revealed that Titan’s upper atmosphere harbors the richest atmospheric organic

chemistry in the solar system (Vuitton et al., 2009a; Waite et al.,2007,2009; Coates et al., 2007; Vuitton

2

et al., 2009b; Sittler et al., 2009; Ali et al., 2010). No other planets and their satellites in our solar system

have been found to bear the level of chemical complexity that we observed on Titan by the Cassini-

Huygens space mission. The Cassini Ion and Neutral Mass Spectrometer (INMS) detection (Waite et al.,

2004) of composition of numerous carbocations (Vuitton et al., 2009a; Waite et al., 2007, 2009) and the

detection of heavy negative ions (up to 13,800 amu/q), in particular very large molecular anions

(carbanions), by the Cassini CAPS Electron Spectrometer (Coates et al., 2007,2009) in Titan’s

thermosphere and ionosphere were two of the highly unexpected findings. Coates et al. (2009) also

found a preference for larger negative ion mass at lower altitude.

Recent studies (Michael et al., 2011) suggest that these larger ions detected (Coates et al., 2007, 2009)

are singly charged, and not super nucleophiles (i.e.,doubly charged negative ions). Michael et al. (2011)

furthermore suggest that molecular charge may play a role in forming larger ions. The larger ions in

Titan’s ionosphere are labeled as “macromolecular” aerosol by Lavvas et al. (2013) rather than by the

familiar term aerosol. These authors (Lavvas et al., 2013) showed that the mechanism of formation of

macromolecules and their growth is directly related to the ion-neutral chemistry. However, the

mechanisms involved in their formation remain largely unclear (Lavvas et al., 2013; Biennier et al.,

2011). A better understanding of the mechanism of formation and growth of macromolecules and their

compositions will require future studies of the molecular structure for the individual carbocations and

carbanions detected by Cassini instruments and of the kinetics of ion-neutral reactions.

In this paper we will focus on the key issue of the chemistry of carbocations (Olah, 1972, 1995) in the gas

phase in space, because their role is now well documented in the reactivity of both saturated

hydrocarbons with single bonds and unsaturated hydrocarbons in many electrophilic reactions in

synthetic organic chemistry. An understanding of the reactions of carbocation chemistry in the gas

phase may shed light on the path toward prebiological molecules-organic inventory in the atmosphere

on Titan.

Among the INMS detected cations, CH5+ is the simplest Olah’s carbocations (Olah, 1972, 1995). Is the

carbocation chemistry in Titan’s upper atmosphere largely governed by Olah’s “nonclassical” carbonium

ions (see below) based structure? The primary theme of “classical” structure of carbo-ions (where the

general concepts of two electron two center (2e-2c) Lewis valence bonds are maintained) versus its

“nonclassical” structure having electron-pair sharing a three-center bond formation was a great

controversial issue, and largely debated some decades earlier. The reader is referred to H. C. Brown’s

monograph, 1977. A series of electrophilic ion-neutral fast reactions as shown below dominate Titan’s

thermosphere and ionosphere. These reactions proceed through carbocation collision complexes over

the potential energy surfaces. The three-center bound nonclassical carbonium ion may lead to the

classical trivalent carbenium global minimum via bond rearrangement. We might therefore seek at a

clear differentiation of classical from nonclassial molecular structures through rotational and rotation-

vibration spectra of carbo-ions. This will bring a major advance in our future understanding of dynamics

and reactivity of carbocations (Vuitton et al., 2009a) detected by Cassini cationic mass spectrometry.

3

The most recent INMS mass spectrometric detection (Vuitton et al.,2009a; Waite et al., 2009) of

protonated methane, CH5+, in Titan’s upper atmosphere is highly intriguing. Its predicted (Muller et al.,

1997; Marx et al., 1997; Schreiner et al., 1993) nonequivalent three equilibrium structures with almost

identical energies and its five protons scrambling bring the question of feasibility of observation of

hypercoordinated carbon in the laboratory (White et al., 1999; Boo et al., 1995; Asvany et al., 2005) and

assigning it to a traditional global minimum molecular structure. Despite a great deal of sophisticated

quantum chemical studies (Muller et al., 1997; Marx et al., 1997; Schreiner et al., 1993; Hinkle et al.,

2011; Witt et al., 2011; Bowman et al., 2010) aimed at understanding of its chemical bonding, and a

number of highly impressive experiments (White et al., 1999; Boo et al., 1995; Asvany et al., 2005;

Huang et al., 2006; Ivanov et al., 2010; and references therein) for obtaining the infrared spectrum for

CH5+, there is not yet a final resolution of its structure and dynamics. Moreover, astronomical

observation of this species in space will undoubtedly require the knowledge of its rotational spectrum

(Bunker et al., 2004). However, the very large amplitude internal motion of this ion will make it

exceedingly difficult to obtain an accurate mm-wave spectrum measured by experiment. One solution to

this problem is perhaps to freeze out the scrambling and internal motions by bringing most of the

populations of this ion to the rotational ground state at very low temperatures under cold and ultra-cold

conditions. As a matter of fact, experimental techniques such as the pulsed CRESU technique (Canosa et

al., 2008) are available to cool complex carbocations such as CH5+ down to 1 K or even below. A

complete understanding of molecular structure and dynamics is though difficult, but certainly feasible if

one attempts to detect the J=1←0 mm-wave absorption transition of isolated CH5+ ion in a

radiofrequency (rf) trap at a temperature close to 1K or further lower (Bunker et al., 2004; Gerlich, 2005)

.

Furthermore, the discover of protonated forms of acetylene, methylacetylene, and dimethylacetylene in

Titan’s upper atmosphere prompt the question whether Olah’s “ nonclassical” structures of carbo-ions

are feasible to observe in space and in the laboratory in the gas phase (Saykally, 1988). The long-

standing pioneering laboratory infrared spectroscopic measurements of Oka ‘s research group for

protonated acetylene clearly establishes that the nonclassical bridged structure of C2H3+ is energetically

the most stable (Crofton et al., 1989). The systematic splittings with the intensity ratio of 2:1 in some

parts of the very rich vibration-rotation spectrum in the 3.2 µm region of the antisymmetric C-H

stretching (ν6 band) have been ascribed to tunneling splittings (Crofton et al., 1989). These tunneling

splittings are due to the apex proton and the two end protons in the bridged structure, which exchange

their positions with a measurable time scale. The primary theme of this infrared spectroscopic

observation is that the nonclassical structure is lower in energy than the classical structure, and the

classical structure is rather a transition state between two equivalent forms of the nonclassical isomers

(Crofton et al., 1989). The transition state barrier height is estimated to be 3.7 ± 1.3 kcal/mol (Lindh et

al., 1991). Concerning the internal motion of protons in this bridged structure of C2H3+, laboratory

measurements of the millimeter and submillimeter wave spectrum of this carbo-ion have been carried

out (Bogey et al., 1992), but there is no indication yet of the observation of tunneling splitting in

rotational transitions in the ground vibrational state.

4

The questions raised above are also applicable to many other carbocation systems such as protonated

methylacetylene C3H5+ and protonated dimethylacetylene C4H7

+. As an example, in analogy to

protonated acetylene, protonated methylacetylene C3H5+ may have nonclassical two-electron three-

center bound bridged structure in addition to its classical counterparts such as the stable 2-methylvinyl

cation. Will the apex proton and the electrophile CH3+ in this bridged structure exchange their positions

within a measurable time scale? In other words the question is whether a nonequivalent equilibrium

carbonium isomer structure will also result where the donation of two electrons from the π bond in

acetylene into the vacant orbital of the apex electrophile CH3+ are delocalized over three centers. The

radioastronomical detection of floppy molecular structures of carbocations in space will require

laboratory measurements of the microwave absorption spectrum of rotational transitions. Once again,

the detection of ultracold ions in rf traps offers a viable route. The observation of nonclassical

“floppy”structures in space is most likely feasible provided that we succeed in laboratory measurements

in bringing most of the populations of these ions in their lowest values of J and K.

The primary focus of this paper is the carbocation chemistry in Titan’s upper atmosphere and the

simplest Huckel Aromatic structure formation thereof. This includes the chemistry of a series of

electrophilic ion-neutral reactions (see below) in Titan’s organic chemistry where π-electron pair from

unsaturated hydrocarbons is donated to the electrophile in the three-center bond. The σ-donation from

saturated hydrocarbons also plays a critical role in forming (2e-3c) nonclassical carbonium structures.

The saturated organic species have in fact the same abilities to share an electron pair from a single bond

in forming delocalized three-center bond formation. The INMS observed both protonated methane CH5+

and protonated ethane C2H7+, clearly supporting this view.

Decades ago, Hoffmann (1964) applied the extended Huckel theory for carbo-ions and predicted a

significant charge delocalization in many classical ion geometries, i.e., the nonclassical carbonium ion

character is preserved in many carbocations. Thus, both classical and nonclassical features will

contribute to the reactivity and chemistry of carbocation in Titan’s upper atmosphere.

Temperatures, densities, and time scales are the three most important parameters that govern the gas

phase reactions involving carbocations and carbanions in Titan’s cold upper atmosphere. The organic

chemistry is also driven by fast neutral reactions (Klippenstein and Georgievskii, 2008; Canosa et al.,

2008) such as radical-molecule reactions. Since the translational energy of molecular species at

temperatures of Titan’s thermosphere and ionosphere is typically of the order of 1 kJ mol-1, only

exothermic reactions with practically no entrance barriers can take place. Even reactions with slight

endothermicity may not occur. Because of low densities, ternary reactions involving three-body

collisions are mostly ineffective. In an elementary reaction the cold and ultracold temperatures prevail

over the electronic and atomic dynamics in a chemical rearrangement process. Thus, the influence of

reagent rotational states in molecular dynamics and reactivity of carbocations is of primary concern.

Ab initio quantum chemical predictions and measurements of rotational and rotation-vibration spectra

at very low temperatures are the basic information required in the astronomical Atacama Large

Millimeter Array (ALMA) interferometric observations of the molecular carbocations detected by INMS

5

as well as in future observations from a submillimeter-wave heterodyne sounder onboard a Titan orbiter

(Lellouch et al., 2010) , and the James Webb Space Telescope (JWST) observations (Gardner et al., 2006).

We note that these spectroscopic observations (see the discussion following) are vital for deriving

molecular structural information on the composition of Titan’s thermosphere and ionosphere revealed

by the Cassini mass spectrometric instruments ( Vuitton et al., 2009a; Waite et al., 2009).

The H3+ ion (Oka, 2006, 2012; Park and Light, 2007; Amano, 2006; Drossart et al., 1989) is the simplest

case with a two-electron three-center bond, and in this sense this ion is the root to the understanding

of the chemistry of electron-deficient carbonium structures (Longuet-Higgins, 1957). The laboratory

infrared spectrum of this ion (Oka, 2006) including millimeter and sub-millimeter observation of its

deuterium-containing isotopologues (Amano, 2006), and its first detection in nature in the infrared

emission bands in the polar auroral regions of the Jupiter ionosphere (Drossart et al., 1989) have

stimulated intensive searches that ended up with its observation in astrophysical plasma universe (Oka,

2006, 2012). This ionic species plays a crucial role in the chemical evolution of nearby interstellar

molecular clouds, the birthplace of stars and planetary systems (Herbst, 2005). Would the carbonium

ion chemistry with similar chemical bonding characteristics in Titan’s thermosphere and ionosphere be

regarded as footprints for organic molecules in the atmosphere of extrasolar planets and young

planetary systems around other stars? It is well acknowledged (Raulin et al., 2009) that organic

chemistry in Titan’s atmosphere is intimately linked to prebiotic organic synthesis in the atmosphere of

primitive earth. The astronomical spectroscopic observations of organic molecules in the atmospheres

of planets as their transit parent stars will most likely place a clear linkage between the chemistry in

Titan’s upper atmosphere and the organic chemistry and aromaticity (see below) in the atmosphere of

young earth.

Ion Neutral Mass Spectrometer (INMS) Observations and Analysis:

Based on INMS Cassini observations (Vuitton et al., 2009a; Waite et al., 2004, 2009, 2007) of specific

carbocations (Olah, 1972, 1995), such as CH3+, CH5

+, C2H3+, C2H5

+, C3H5+, and C3H7

+, and on the analysis of

previous laboratory crossed molecular beam measurements (Sonnenfroh and Farrar, 1986) and ab initio

theoretical studies (Lopez et al., 1996; Almlof et al., 1984; Hariharan et al., 1974) of the reactions of CH3+

with ethylene and acetylene, in this paper we infer (see discussion below) that a significant fraction of

the detected C3H3+ composition (Vuitton et al., 2009a; Waite et al., 2009) by INMS in Titan’s upper

atmosphere is the stable Cyclopropenyl Cation - the simplest Huckel’s Aromatic Molecule. What

naturally follows is also the identification of an important class of reactions in Titan’s upper atmosphere

such as those of CH3+ with methylacetylene and dimethylacetylene. The latter offers an explanation of

the observed composition of C4H5+ in terms of methyl substituted cyclopropenyl cation and of C5H7

+

where two hydrogen atoms in the cyclopropenyl cation are substituted by two methyl groups (Vuitton

et al., 2009a; Waite et al., 2007).

6

In Titan’s ionospheric chemistry, both C2H3+ and C2H5

+ are the protonated forms of two abundant

neutrals: acetylene and ethylene, respectively (Vuitton et al., 2009a; Waite et al., 2005, 2009). These

ions are primarily formed by proton transfer reactions (Vuitton et al., 2009a) to the corresponding

neutrals from ionic hydrocarbon species with smaller proton affinities. Below are shown four important

ion neutral exothermic reaction channels (Sonnenfroh and Farrar, 1986; Lopez et al., 1996) that could be

accessible at the low temperatures (i.e., from 10 to 200 K) typical of interstellar clouds and planetary

atmospheres:

CH3+ + C2H4 → C2H3

+ + CH4 ∆H = -1.02 eV, (1)

→ C3H5+ + H2 ∆H = -2.11 eV, (2)

→ C3H3+ + 2H2 ∆H = -0.85 eV, (3)

CH3+ + C2H2→ C3H3

+ + H2, ∆H = -2.65 eV, (4)

Both reactions presented above are known to involve condensation reactions proceeding through the

formation of transient carbocation collision complexes. Ab initio (Lopez et al., 1996; Almlof et al., 1984;

Hariharan et al., 1974) theoretical studies of the reaction paths and crossed beam measurements

(Sonnenfroh and Farrar, 1986) of the products center of mass angular flux and translation energy

distributions have been carried out to determine branching ratios of the product formation from the

reaction of CH3+ with ethylene, as well as the mechanism of formation of C3H3

+ from reaction (4)

exclusively. We do note however that these crossed beam measurements (Sonnenfroh and Farrar, 1986)

have been performed at collision energies between 0.5 and 2.0 eV. These initial energies are much

higher than the average collision energy at the low temperatures prevailing in Titan’s upper

atmosphere.

The measurements of products center of mass angular flux distributions are usually indicative of the life

times (a fraction of a rotational period) of short lived complexes participating in bimolecular reactions.

Such studies have allowed (Sonnenfroh and Farrar, 1986) to construct a reaction pathway that leads to

the initial formation of three –center two-electon bond (Olah, 1972, 1995; Longuet-Higgins, 1957) in

metastable corner protonated cyclopropane (C3H7+) by an electrophilic attack of CH3

+ to the π cloud of

ethylene. Ab initio calculations of reation channels indicate (Sonnenfroh and Farrar, 1986; Almlof et al.,

1984; Hariharan et al., 1974) that C3H7+ readily isomerizes by ring opening to the n-propyl cation,

followed by a 1,2 hydrogen atom shift to the lowest energy sec-C3H7+ structure. Decay (Sonnenfroh and

Farrar, 1986) to CH4 + C2H3+ (hydride abstraction) from the sec-C3H7

+ minimum structure is endothermic

by 60 kcal mol-1. In the literature no estimates are provided so far for what concerns the magnitude of

the exit channel barrier for this process in reaction (1). The angular flux contour maps (Sonnenfroh and

Farrar, 1986) for allyl cation (C3H5+) formation have some similarities to those for the CH4 reaction

channel (1) and this suggests that both reaction channels (1) and (2) have a common precursor collision

complex. Ab initio calculations show (Sonnenfroh and Farrar, 1986; Almlof et al., 1984) further that

formation of the lowest energy allyl cation from the sec-C3H7+ intermediate in reaction channel (2)

proceeds over a significant exit barrier of approximately 51 kcal mol-1. This is consistent (Sonnenfroh and

7

Farrar, 1986; Almlof et al., 1984) with the metastable mass spectrometric measurements (Burgers et al.,

1983) of the kinetic energy release in the fragmentation of C3H7+ and the reported product center of

mass translational energy distribution (Sonnenfroh and Farrar, 1986) for C3H5+. In passing we do note

the INMS detection of the composition of the stable allyl cation C3H5+.

The mechanistic interpretation of the cyclic-C3H3+ isomer formation in the reaction channel (3) is drawn

from the kinematics of crossed beam measurements (Sonnenfroh and Farrar, 1986). The reaction

channel (3) above requires the loss of two molecules of hydrogen from the metastable adduct sec-C3H7+

formed from the colliding reagents. Is this process simultaneous or sequential? If the process of ejection

of two molecules of hydrogen from the adduct occurs in a single event, the determination of the

kinematics would be notoriously difficult, indeterminate. It is therefore assumed (Sonnenfroh and

Farrar, 1986) that the initial complex C3H7+ sequentially loses a single H2 molecule, and the resultant

internally excited C3H5+ forms C3H3

+ from the loss of another molecule of hydrogen in the reaction

process (3). Because of the large difference in mass between H2 and the unobserved C3H5+ in reaction

channel (3), and since only highly internally excited C3H5+ will dissociate to form C3H3

+, it is reasonable to

assume that the new center of mass velocity vector lies along the initial center of mass of the colliding

reagents. In essence, the final hydrogen molecule is emitted relative to the original centroid of the

collision system. This assumption allows (Sonnenfroh and Farrar, 1986) to extract from crossed beam

measurements product center of mass angular flux and translational energy distributions for C3H3+

formation from the reaction of CH3+ with C2H4 at various collision energies. The measurements

(Sonnenfroh and Farrar, 1986) of angular flux distributions reflect that C3H3+ formation proceeds by

sequential ejection of two hydrogen molecules from the initial C3H7+ collision complex. It is important to

note that the widths of the translational energy distribution (Sonnenfroh and Farrar, 1986)

measurements for C3H3+ formation require that a significant fraction of the C3H3

+ products must be the

more stable cyclopropenium isomer. Ab initio studies (Lopez et al., 1996) show that the exit channel for

the reaction of CH3+ with acetylene to form C3H3

+ involves similar C3H5+ intermediates. Therefore, a

discussion for the reaction (4) leading to the formation of both cyclic isomer and linear propargyl cations

turns out to be necessary. Fig.1 represents a schematic reaction coordinate energy profile for the CH3+ +

C2H2 system (Sonnenfroh and Farrar, 1986; Lopez et al., 1996; Almlof et al., 1984; Hariharan et al., 1974).

8

Fig. 1. Schematic potential energy surface for the CH3++C2H2 system. The delocalized Allyl

cation is the global minimum. Both products cyclic-C3H3+ and linear-C3H3

+ are shown along

with the isomerization barrier separating them. The larger circles in structures denote carbon

atoms. Three collision complexes and nine transition states are schematically shown in the

reaction coordinate plot. The numbers denote zero-point corrected energies (in Kcal/mol) relative

to the reactants.

C3H3+ has two isomers: the cyclopropenyl cation and the propargyl cation. The former is approximately

27.4 kcal/mol more stable than the second one (Lopez et al., 1996). The formation of cyclopropenyl

cation + H2 (~ -61.7 kcal/mol) as well as the formation of propargyl cation + H2 (~ -34.3 kcal/mol) are

both exothermic processes in the reaction channel (4) listed above. The reaction of CH3+ with acetylene

proceeds through the short-lived C3H5+ intermediate which has three different isomeric forms (Lopez et

al., 1996): (1) the corner-protonated cyclopropene (-61.0 kcal/mol) where CH3+ is directly above the

carbon-carbon double bond forming two-electron three-center bonding, (2) the 2-methylvinyl cation [

CH3-C=CH2]+ (-79.2 kcal/mol), and (3) the stable delocalized allyl cation [CH2=CH-CH2]

+ (-89.5 kcal/mol) -

the global minimum. Besides these three intermediate collision complexes, recent theoretical

calculations (Lopez et al., 1996) have located on the potential energy surface various transition state

structures more stable than the reactants of C3H5+ that finally lead to the formation of both isomers of

9

C3H3+. In Fig. 1, the energies shown in brackets are relative to the reactants. The calculations (Lopez et

al., 1996) showed that the reactants in channel (4) lead directly to either the corner–protonated

cyclopropene or the 2-methylvinyl cation, without encountering any energy barrier. In the entrance

channel (see Fig.1), the transition state structure between the corner-protonated cyclopropene and the

2-methylvinyl cation corresponds to an energy barrier of only 1.3 kcal/mol. Similarly, the transition state

between the corner-protonated cyclopropene and the allyl cation corresponds to an energy barrier of

approximately 10.4 kcal/mol. The transition structure between the most stable intermediate allyl cation

and the 2- methylvinyl cation corresponds to an energy barrier of 31.2 kcal/mol. Ab initio calculations

(Lopez et al., 1996) also pointed out that both the cyclic-C3H3+ and linear-C3H3

+ isomers are generated in

the unimolecular decomposition of the collision complex C3H5+ in the bimolecular reaction of CH3

+ with

acetylene. The reaction coordinate calculation (Lopez et al., 1996) identifies in the exit channel three

transition state structures: The lowest energy transition state links the corner-protonated cyclopropene

to cyclopropenyl cation + H2 products with an energy barrier of 33.3 kcal/mol, with the hydrogen

molecule being formed from two H atoms of the CH3+ moiety of the transition state (Fig.1). The other

two saddle points (Lopez et al., 1996) lead to the propargyl cation and H2 from the collision complex 2-

methylvinyl cation. In one of the transition structures the hydrogen molecule is formed from two H

atoms of the CH3+ moiety with an energy barrier of 55.1 kcal/mol, while in the other transition state the

hydrogen molecule is formed from one H atom of the CH3+ moiety and one H atom of a CH group with

an energy barrier of 57.3 kcal/mol.

The key question is the isomeric identity of C3H3+ produced from the reaction of CH3

+ with ethylene and

acetylene in Titan’s upper atmosphere. As indicated earlier, the cyclic isomer is approximately 27.4

kcal/mol more stable than the linear propargyl cation. The intramolecular isomerization leading to

product isomers occurs over a barrier of 50 kcal/mol relative to the propargyl (Fig.1). The total energy

available to products in a reaction depends upon reagent internal excitation, relative translational

energy and reaction exothermicity. How many C3H3+ isomers are internally excited over their

corresponding barrier heights to interconvert? We note that the reaction of CH3+ with acetylene is much

more exothermic than that with ethylene. The products in the reaction channel (3) are almost

isoenergetic (or slightly above in the case of propargyl cation) to the reactants (Sonnenfroh and Farrar,

1986), whereas in channel (4) the products are well below the energy of the reactants (Sonnenfroh and

Farrar, 1986; Lopez et al., 1996). The thermochemical limits for propargyl and cyclopropenyl cation

formation show that the widths of product center of mass translational energy distributions in the

crossed beam measurements (Sonnenfroh and Farrar, 1986) bear on the branching issue between cyclic

and linear C3H3+. With decreasing the relative collision energy of the reagents from 1.15 eV to 0.49 eV,

the changes in widths of product translational energy distribution for C3H3+ products in the reaction of

CH3+ with acetylene reflect that the maximum energies available to surmount the transition barrier to

propargyl formation decreases substantially (Sonnenfroh and Farrar, 1986). The widths of the kinetic

energy distribution at such low collision energies require that a significant fraction (at least 40%) of

products is the most stable cyclic isomer (Sonnenfroh and Farrar, 1986). In the experiment in the

relative reagent collision energy from 0.5 to 2.0 eV, the lowest kinetic energies for C3H3+ isomers

correspond to the most highly internally excited products with excitation above the isomerization

10

barriers separating the linear and cyclic isomers. Such products are intrinsically unassignable in the

experiment to their isomeric form (Sonnenfroh and Farrar, 1986). The translation energy distributions

for the C3H3+ product of the ethylene reaction have been viewed in the same way as those for the

acetylene reaction (Sonnenfroh and Farrar, 1986). With decreased reagents relative collision energy in

the experiment (Sonnenfroh and Farrar, 1986), the formation of cyclopropenyl cation- the cyclic isomer

is more favored.

The temperature at which the various chemical reactions in the gas phase take place in Titan’s upper

atmosphere is close to 130 K. The temperature could be as low as 10 K for ion chemistry in interstellar

clouds. The initial collision energies in the crossed beam reactive scattering experiments (Sonnenfroh

and Farrar, 1986) are much higher by several orders-of-magnitude than the average collision energies at

the low temperatures (10-200 K) prevailing in planetary atmospheres and interstellar dense clouds. With

respect to temperature dependent kinetic rates and dynamics proceeding over the potential energy

surfaces, there are limitations in the current measurements (Sonnenfroh and Farrar, 1986; Burgers et

al., 1983). The exact determination of the product branching ratio, e.g., linear C3H3+ versus cyclic C3H3

+

strongly depend upon collision energy. We emphasize the importance of the entrance and exit inner

transition state barriers (Sonnenfroh and Farrar, 1986; Lopez et al., 1996) in the reactions of CH3+ with

ethylene and acetylene mentioned above. In these ion-molecule reactions, in the entrance channel the

transition states connecting collision complexes to the covalently bound global minimum all lie well

below the reactants (Sonnenfroh and Farrar, 1986; Lopez et al., 1996; Almlof et al., 1984). Thus, simple

capture rates (Clary, 1987; Klippenstein and Georgievskii, 2008) in the limit of long range attractive

forces can provide an accurate description of the kinetics of the formation of ion-molecule complexes at

higher energies around room temperature as well as at the lower collision energies prevailing in

planetary atmospheres and interstellar clouds. In treating the unimolecular decomposition of

intermediate collision complexes to the products (Wardlaw and Marcus, 1988; Gilbert and Smith, ed.

1990; Holbrook, Pilling, and Robertson, ed. 1996), multiple transition states in the exit channel are

involved (Sonnenfroh and Farrar, 1986; Lopez et al., 1996), though they are seldom of kinetic

importance. The products in the potential energy surface (Lopez et al., 1996) for the CH3+ + C2H2 system

are linear-C3H3+ + H2 and Cyclic-C3H3

+ +H2. The exit channel barrier (Lopez et al., 1996) in the potential

energy surface for this system connecting the products linear-C3H3+ + H2 with the 2-methylvinyl cation

covalently bound collision complex is approximately 21.9 kcal/mol below the energy of the reactants,

whereas the barrier of the exit saddle point that connects the products cyclic-C3H3+ + H2 with the corner-

protonated cyclopropene intermediate is approximately 27.7 kcal/mol below the reactants in energy.

We thus expect the formation of both linear and cyclic isomers, with the cyclopropenyl cation as the

more favored product (see Fig.1). This is in line with what discussed above, i.e., that the cross beam

measurements (Sonnenfroh and Farrar, 1986) of center of mass translational energy distributions for

C3H3+ reflect the product branching ratio of cyclic to linear increases with the decrease of reagent

relative collision energy from 2.0 to 0.5 eV. The dominance of long-range attractive potential (Clary,

1987; Klippenstein and Georgievskii, 2008) in the entrance channel and the multiple transition state

models (Lopez et al., 1996) in the product exit channel thus provide clear evidence of the formation of

the cyclopropenyl cation, as the major product in the conditions of Titan’s upper atmosphere and

11

interstellar clouds at low temperatures (10-200 K). The process of isomerization between linear and

cyclic isomer of C3H3+ does not occur at temperatures relevant to the chemistry of Titan’s atmosphere,

because of a very large barrier of about 50 kcal/mol relative to the linear isomer (Fig.1). The

isomerization barrier is higher in energy than that to the reactants.

Similarly, the entrance and exit barriers (Sonnenfroh and Farrar, 1986) in the CH3+ + C2H4 reaction

system determine the product branching ratios at low temperatures (10-200 K). Several transition states

have been identified in the exit channel of this reaction. These transition states were shown

schematically (Sonnenfroh and Farrar, 1986) and somewhat empirical for three product channels, C2H3+

+ CH4, C3H5+ + H2, and C3H3

+ + 2H2. The saddle point in the entrance of the potential energy surface

connecting the corner-protonated cyclopropyl cation (C3H7+) intermediate (Almlof et al., 1984; Hariharan

et al., 1974) to the covalently bound short-lived sec-C3H7+ structure tends to lie (Sonnenfroh and Farrar,

1986) well below the energy of the reactants, because of long-range increased attractions in these ion-

molecule complexes. In this respect, simple capture theory (Clary, 1987; Klippenstein and Georgievskii,

2008) provides an accurate description of the long-range part of this attractive potential.

The hydride abstraction reaction or the formation of the product C2H3+ + CH4 via unimolecular

decomposition (Wardlaw and Marcus, 1988; Gilbert and Smith, ed. 1990; Holbrook, Pilling and

Robertson, ed. 1996) of sec-C3H7+ in the exit channel does not involve (Sonnenfroh and Farrar, 1986) any

transition state barrier. Again, the barrier to the formation of lowest energy allyl cation (C3H5+) plus the

hydrogen molecule from the complex sec-C3H7+ is approximately 30 kcal/mol below the energy of the

reactants (Sonnenfroh and Farrar, 1986). The isomer products linear- C3H3+ + H2 are very close in energy

or slightly above to that of the reactants. The cyclic-C3H3+ + H2 are found to be ~ 20 kcal/mol below the

reactants. There are two large exit channel barriers in the schematic potential energy surface

(Sonnenfroh and Farrar, 1986) of sec-C3H7+ for producing both linear and cyclic isomers of C3H3

+ from the

internally excited intermediate allyl cation (C3H5+). Because the products cyclic-C3H3

+ + H2 are in energy

well below the reactants CH3+ + C2H4, and the exit channel barrier of this reaction does not cross the

thermochemical limit, we expect the cyclic isomer is the most dominating form for C3H3+ at relevant low

temperatures. Indeed, a very small exit channel barrier does not allow an efficient formation of the

linear-C3H3+ + H2 in the chemistry of atmosphere of Titan and interstellar clouds. To elaborate this

specific point and to determine temperature dependent reaction rate coefficients in such complex-

forming ion-molecule reactions at low temperatures (10-200 K), ab initio electronic structure

calculations and microcanonical variational RRKM transition state theory (Wardlaw and Marcus, 1988;

Gilbert and Smith, ed. 1990; Holbrook, Pilling, and Robertson, ed.1996) are required in future studies.

Titan’s upper atmosphere is chemically complex. The reactions involving neutrals, ion-molecule

reactions and the chemistry of negative ions initiate this complexity (Vuitton et al., 2009a, 2009b; Waite

et al., 2009; Coates et al., 2007; Sittler et al., 2009; Ali et al., 2010; Klippenstein and Georgievskii, 2008;

Balucani, 2009; Larsson et al., 2012) and start the organic inventory in the second largest moon of our

solar system. No other planetary environments in our solar system are known to exceed such chemical

complexity. The observation of INMS mass spectrum (Vuitton et al., 2009a; Waite et al., 2007) of two

coupled ions (C4H7+, C4H5

+), and (C5H9+, C5H7

+), and the preceding analysis of the energetics as well as the

12

dynamics proceeding over the potential energy surface for the CH3+ + C2H2 system force us to predict the

following “complex-forming” bimolecular reactions of CH3+ with methylacetylene and dimethylacetylene

in Titan’s upper atmosphere:

CH3+ + HCCCH3 → CH3C3H2

+ + H2 (5)

CH3+ + CH3CCCH3 → CH3C3HCH3

+ + H2 (6)

The reaction (5) is expected to yield molecular hydrogen and the cyclopropenyl cation with one of the

hydrogen atoms substituted by a methyl functional group. According to the discussion above, one also

expects the formation of the linear isomer- methyl propargyl cation [CH(CH3) = C = CH]+ in this

“condensation reaction” channel. In the reaction (6), the products are the hydrogen molecule and the

cyclic isomer of C3H3+ where two hydrogen atoms are replaced by methyl groups. Similarly, the linear

isomer [C(CH3)2 = C = CH]+ is also formed in the reaction of CH3+ with dimethylacetylene. Both reactions

belong to the same condensation reaction class. To the best of our knowledge, the energetics and

potential energy characteristics for the ion-molecule reactions (5) and (6) are not known. There are not

any laboratory studies of the lifetimes of the collision complexes involved in these two exoergic

reactions that can provide the dynamical information and product isomer branching ratios.

Methylacetylene is an abundant molecule (Waite et al., 2009) in Titan’s thermosphere and ionosphere.

It is a polar molecule and thus we would expect an increased strength in the long-range force (Clary,

1987; Klippenstein and Georgievskii, 2008) in the interaction of a closed-shell ion and a polar molecule

in channel (5). Because of this strong long-range interaction, the saddle points connecting ion-molecule

complexes to the covalently bound global minimum are expected (Klippenstein and Georgievskii, 2008)

to lie well below the reactants in the attractive part of the potential energy surface. We therefore

suggest that the simple capture theory (Clary, 1987; Klippenstein and Georgievskii, 2008) is able to

provide an accurate description of this ion-dipole reaction kinetics, provided that the potential energy

characteristics in the exit channel of the reaction are very similar to that of the CH3+ + C2H2 system.

Similar to the attack of CH3+ to acetylene, the ion-molecule reaction (5) forms the C4H7

+ collision-

complex which, via isomeric rearrangement, leads to the global minimum- the delocalized allyl cation

(Lopez et al., 1996) where one of the hydrogen atoms in either of the two end carbon atoms is replaced

by a methyl group [ CH2 = CH – CH(CH3) ]+. In addition to the lowest energy minimum, the C4H7

+ complex

has two other higher energy isomers: the corner-protonated methylcyclopropene and the 2-methylvinyl

cation (Lopez et al., 1996) where one hydrogen of the vinyl cation is replaced by a methyl functional

group [ CH3 – C = CH(CH3) ]+. Similarly to the reaction of CH3

+ with acetylene, both higher energy isomers

of the collision complex C4H7+ likely evolve to the corresponding products, the cyclic isomer CH3C3H2

+ +

H2 and the linear isomer [CH(CH3) = C = CH]+ +H2, through transition state structures. It is the corner-

protonated methylcyclopropene intermediate that is linked to the cyclic isomer of the product in the

exit channel of reaction (5), whereas the intermediate structure [ CH3– C = CH(CH3) ]+ is connected to the

linear product isomer. The lifetime of the intermediate transient complex is only a fraction of the

rotational period. As the temperature decreases from room temperature down to 10 K, the decrease of

the average collision energy increases the lifetimes of transient complexes (Wardlaw and Marcus, 1988;

13

Gilbert and Smith, ed. 1990; Holbrook, Pilling and Robertson, ed. 1996). Because of this increased

lifetime, the intermediate complexes in such reactions at low temperatures evolve to the potential

global minimum such as C4H7+ and to the methylallyl structure [ CH2 = CH – CH(CH3) ]

+.

On a similar basis, if we assume that the potential energy characteristics of the system CH3+ + CH3CCCH3

are similar to that of the CH3+ + C2H2 system with planar structure of dimethylallyl cation as the global

minimum for the intermediate complex C5H9+, in the reaction channel (6) we expect as well the

formation of both products, the cyclic isomer dimethylcyclopropenyl cation CH3C3HCH3+ and its linear

counterpart dimethyl propargyl cation [C(CH3)2 = C = CH]+.

The key issue we have presented here is the synthesis of the cyclopropenyl cation, the simplest Huckel’s

aromatic molecule, and its cyclic methyl derivatives. Carbocations observed by the INMS (Waite et al.,

2004, 2007, 2009; Vuitton et al., 2009a) play a dominant role in the chemistry of Titan’s upper

atmosphere. This role starts from the formation of penta- or tetracoordinated “nonclassical” carbonium

(Olah, 1972, 1995) collision complexes which, via isomeric rearrangement, lead to the stable C3H7+,

C3H5+, C4H7

+, and C5H9+ systems- the global minima on the potential energy surfaces of the reactions

involving CH3+ with ethylene, acetylene, methylacetylene, and dimethylacetylene, respectively. An

understanding of temperature dependent kinetic rate coefficients will ultimately require the

microcanonical variational RRKM transition state theory (Wardlaw and Marcus, 1988; Gilbert and Smith,

ed. 1990; Holbrook, Pilling, and Robertson, ed. 1996) analysis of both unimolecular decomposition and

bimolecular recombination reactions in the C3H7+, C3H5

+, C4H7+, and C5H9

+ systems. It is shown here that

the product branching ratios of cyclic to linear isomers of C3H3+ and its methyl derivatives are strongly

temperature dependent (10-200 K), and as the temperature of the system decreases down to the

temperature of atmospheres of outer planets and their satellites and interstellar clouds, the kinetics of

formation of cyclic isomers is dominant. It is further pointed out that in the low temperature limit the

kinetics of the reaction of a closed shell ion with a polar or a non-polar molecule in the gas phase can be

determined by the capture rate (Klippenstein and Georgievskii, 2008) on the long-range potential, unless

the transition state bottlenecks are almost equal in energy or above to the reactants. The simple

classical or quantum mechanical capture theory (Clary, 1987; Klippenstein and Georgievskii, 2008)

provides a fair description of the chemical reaction rate coefficients at low temperatures of these fast

reactions.

Future Studies and Concluding Remarks:

The detection of a series of carbocations (Waite et al., 2009; Vuitton et al., 2009a) and carbanions

(Coates et al., 2007; Vuitton et al., 2009b; Sittler et al., 2009; Ali et al., 2010) in Titan’s upper

atmosphere by instruments onboard the Cassini mission (Waite et al., 2004; Young et al., 2004) has a

very special significance in ion chemistry in space. We reiterate that, among the detected carbocations,

CH5+ represents the simplest of Olah’s (Olah, 1972, 1995) carbocations, and currently this is of

considerable interest in space science (Huang et al., 2006; Asvany et al., 2005; McCoy et al., 2004;

14

Muller et al., 1997; White et al., 1999). Olah’s (Olah, 1972, 1995) three-center two-electron penta- or

tetracoordinated “nonclassical” carbonium ions based structure of carbocations is no longer a

controversial topic in chemistry. The quantum-mechanical treatment of electron-deficient bonding

among three attracting centers of charge was first introduced by Longuet-Higgins, 1957. Many

theoretical studies have been published on this subject since that time. Nevertheless, a number of

nonequivalent equilibrium structures for CH5+ with almost identical energies makes its rotational and

vibrational-rotational spectra unusually complicated (White et al., 1999; Bunker et al., 2004). It is

perhaps the millimeter-wave laboratory absorption spectroscopy of specific rotational transitions of

“sufficiently cold” mass-selected CH5+ ion in ion traps (Gerlich, 2005; Ghosh, 1995) that can eventually

lead to the astronomical observational confirmation of this fascinating (Olah, 1995) cation, which is

predicted to be so abundant in space (Waite et al., 2009; Vuitton et al., 2009a).

The Cassini INMS instrument (Waite et al., 2004) is in essence a quadrupole mass spectrometer. In the

upper atmosphere of the moon it measures the mass distributions of a variety of neutrals and positive

ions up to 100 amu with one atomic mass unit resolving power between masses (Waite et al., 2005,

2009; Vuitton et al., 2009a). It thus provides primarily information on the chemical compositions. Then,

the molecular structures corresponding to the INMS detected composition require spectroscopic

features as fingerprints. Thus, there is an urgent need of direct ground-based or in situ spectroscopic

observations in the infrared and millimeter-wave region.

Since cyclic-C3H3+ has a D3h symmetry, it has no permanent dipole moment. Thus, searching this

molecule in the atmosphere of Titan by means of pure rotational spectroscopy is not an option.

However, various isotopologues of cyclic-C3H3+ via inclusion of one 13C or one D atom possess a

permanent dipole moment (Waite et al., 2005; Abbas et al., 2010). Ab initio quantum-chemical

calculations (Huang and Lee, 2011) have recently been performed to compute fundamental vibrational

frequencies and spectroscopic constants for rotational and vibro-rotational spectra for the c-13CC2H3+, c-

C3H2D+, c-13CC2H2D

+ isotopologues of cyclic-C3H3+, and the H2CCCD+, HDCCCH+, H2

13CCCH+, H2C13CCH+, and

H2CC13CH+ isotopologues of linear-C3H3+. The predicted dipole moment values (Huang and Lee, 2011) for

the isotopologues of the cyclic isomer noted above are 0.094, 0.225, and 0.312 D, respectively, while the

linear isotopologues have values that range from 0.325 to 0.811 D. Fig.2 depicts the predicted rotational

spectra (a-type) for c-C3H2D+, c-13CC2H2D

+ and l-H2CCCD+ in the 20-150 GHz frequency range, based on

the spectroscopic parameters from Huang and Lee, 2011. We do note that the linear isotopic species,

having a dipole moment of 0.811 D and a large rotational constant A, presents a spectrum definitely

different from that of the cyclic mono-deuterated isotopologues considered. More interesting is to note

that rather different are also the rotational spectra of the two cyclic isotopic species. In fact, despite of

having similar dipole moments (0.225 and 0.312 D) and similar rotational parameters (for instance, their

B and C values differ by only a few %), the two rotational spectra are such that allow a clear

identification of the two isotopologues. This is well demonstrated by the J = 32,2←22,1 transition shown in

the inset of Fig.2: the corresponding frequencies differ by about 3 GHz. Due to the large difference in A,

for the linear species the same transition lies in a complete different region of spectra. State-of-the-art

ab initio quantum-chemical methods have reached such a level that theoretical chemists are able to

15

compute very accurate spectroscopic parameters for rotational spectroscopy of small- to medium-sized

molecules (Puzzarini et al., 2010). The interplay among theory (Puzzarini et al., 2010), millimeter-

/submillimeter-wave laboratory measurements (Cazzoli and Puzzarini, 2008; Savage and Ziurys, 2005;

Cazzoli et al., 2012), and astronomical observations of rotational transitions leads to the most powerful

tool to derive molecular structural information for the composition of carbocations (Vuitton et al.,

2009a; Waite et al., 2009) determined by Cassini neutral and cationic mass spectrometry (Waite et al.,

2004).

Fig. 2. Calculated pure rotational spectra for 13

C and deuterium-containing isotopologues of

cyclic and linear C3H3+ in the 20–150 GHz frequency range.

One seminal advance in the field of rotational spectroscopy was the demonstration by Woods and his

coworkers that it was possible to observe the rotational spectra of molecular ions at high resolution in a

DC discharge plasma (Woods et al., 1975). More recently (Savage and Ziurys, 2005), AC discharges have

been used that make it possible to use the velocity modulation (Gudeman and Saykally, 1984;

Stephenson and Saykally, 2005) technique (extensively used in the rovibrational spectroscopy) to

separate spectral lines due to ions from those arising from neutral species, thus rendering rotational

16

spectroscopy (in the millimeter/submillimeter frequency ranges ) well suited for studies of carbocations.

Another important advance in the investigation of the rotational spectra of ions was the employment of

an axial magnetic field to extend the ion rich negative glow region of an abnormal glow discharge, thus

leading to an increase of ion column densities by about a factor of 100 (De Lucia et al., 1983). This

approach has been widely adopted for observations of molecular cations in the laboratory, thus making

it possible their identification in the interstellar medium. The biggest hurdle (Ricks et al., 2010; Garand

et al., 2008) in infrared and microwave spectroscopy experiments is to selectively study a sufficiently

cold isomer of a molecular ion in an isolated condition, even if it is well demonstrated that rotational

spectrometers based on jet expansion and Fourier Transform technique are able to characterize

different conformers at the same time (Cocinero et al., 2012; Pena et al., 2012). As an example, the

C4H5+ potential energy surface (Cunje et al., 1996), as evaluated by ab initio computations, is highly

complex. A number of minima and several transition structures permitting interconversion between the

minima have been located on the potential energy surface. The methylcyclopropenyl cation is the global

minimum (Cunje et al., 1996), and five other isomers are above the global minimum by 9.1 to 26.9

kcal/mol. Because of the existence of multiple minima on the potential energy landscape and substantial

barriers to the isomerization processes, each isomer is trapped at the bottom of its corresponding

minimum well without the possibility to convert to the lowest energy geometry. As a result, in

methylcyclopropenyl cold cation synthetic scheme, the presence of multiple isomers is evident. In an

ideal situation, one wants to study infrared or microwave absorption spectra of cold ions (such as C4H5+)

at their lowest energy geometry in rf traps (Garand et al., 2008; Ghosh, 1995). An extension of the

absorption spectroscopy technique in molecular structural elucidation is feasible, provided that we find

a methodology where the kinetics of the branching ratios of product isomers formation for a given set of

reactions can be controlled as the temperature is varied at low temperatures (10-200 K). It is an area

where the analysis of spectroscopic properties of equilibrium structures of carbocations requires ab

initio quantum-chemical calculations extensively. We expect high-level quantum-chemical methods

(Puzzarini, et al., 2010) to be able to compute precisely fundamental vibrational frequencies, rotational

constants, vibration-rotation interaction constants and other relevant spectroscopic parameters for the 13C and deuterium-containing isotopologues of the methylcyclopropenyl and dimethylcyclopropenyl

cations. Furthermore, unlike the case of charged cyclic-C3H3+, in both methyl and dimethyl cyclopropenyl

cations the center of nuclear charge and the molecular center of mass are no longer the same. It is thus

hoped that the laboratory measurements together with the theoretical study of the infrared and

millimeter/submillimeter-wave spectra of isotopologues of cyclic-C3H3+ and its methyl derivatives will

enable their identification in astronomical observations by means of interferometric arrays such as the

Atacama Large Millimeter Array (ALMA), and in the distant future, from a submillimeter-wave

heterodyne sounder onboard a Titan orbitter (Lellouch et al., 2010), and from the James Webb Space

Telescope (JWST) expected to operate (Gardner et al., 2006) in the near and mid infrared wavelength

range between 0.6 and 29 µm.

Formation of Huckel rings and introduction of aromatic character in cyclic compounds are considered to

be the most important classes of reactions in Titan’s organic chemistry (Vuitton et al., 2009a; Waite et

al., 2009; Sittler et al., 2009). These simplest Huckel’s cyclic molecules stabilized by aromaticity play a

17

pivotal role in carbocation chemistry in Titan’s upper atmosphere. Because of the importance of the

chemical pathways for the formation of these aromatics, laboratory and theoretical studies of kinetics

and dynamics of specific elementary reactions in the gas-phase such as reactions of CH3+ with ethylene,

acetylene, methylacetylene and dimethylacetylene at relevant low temperatures (10-200 K) are crucial.

Two major advances in experimental techniques (Dupeyrat et al., 1985; Rowe et al., 1984; Smith and

Rowe, 2000; Lee, 1987; Balucani, 2009) are now available to characterize the kinetics and dynamics of

these elementary reactions in the gas-phase. The first one involves the CRESU (Cinetique de Reaction en

Ecoulement Supersonique Uniforme, or Reaction Kinetics in Uniform Supersonic Flow) technique (Rowe

et al., 1984; Dupeyrat et al., 1985) that allows the measurements of reaction rate constants at selected

low temperatures (10- 200 K). Second, the crossed beam study (Lee, 1987; Balucani, 2009) of ion-

molecule reactions (Sonnenfroh and Farrar, 1986), especially at very low collision energies, can

complement with an important piece of information on the reaction products and their branching ratios.

Most importantly, the crossed beam studies probe the reaction dynamics proceeding over potential

energy landscape, thus confirming a specific reaction pathway and whether its related products are

accessible at low temperatures. Finally, quantum-chemical calculations of energetics and variational

RRKM transition state theory (Wardlaw and Marcus, 1988; Gilbert and Smith, ed. 1990; Holbrook, Pilling,

and Robertson, ed. 1996) of unimolecular dissociation and bimolecular recombination kinetics of the

collision complexes C3H7+, C3H5

+, C4H7+, and C5H9

+ in the reactions mentioned above are needed to

provide a mechanistic interpretation of the formation of the simplest form of aromatic cyclic rings in

Titan’s ionospheric chemistry. As already pointed out, in the low temperature limit the kinetics of gas

phase reactions of closed-shell ions with polar and non polar molecules can be well characterized by

classical or quantum mechanical simple capture rate (Clary, 1987; Klippenstein and Georgievskii, 2008)

on the long-range potential, as the transition states in the entrance channel are well below in energy

with respect to the reactants (Sonnenfroh and Farrar, 1986; Lopez et al., 1996).

The formation of complex organic molecules in the interstellar medium proceeds via ‘abiotic’ synthesis

(Klemperer, 2011). The chemistry in Titan’s upper atmosphere may tell us a lot about the abiotic model

of synthesis of organics in interstellar clouds. The details of processes in Titan’s ionospheres differ from

ion-molecule reactions in the interstellar medium in a subtle manner (Larsson et al., 2012; Klemperer,

2011). The cyclopropenyl cation C3H3+ has not yet been detected in interstellar clouds. For the formation

of C3H3+, the radiative association reaction of C3H

+ with H2 at 10 K has been proposed (Klemperer, 2011)

in interstellar clouds, where H2 is by far the most abundant molecular species. A quantitative

understanding of the radiative association rate coefficients (Smith, 1989; Brownsword et al., 1997;

Vuitton et al., 2012) is far from being complete. There is an urgent need of an experiment (Gerlich and

Horning, 1992) on radiative association rates of C3H+ with H2 at low temperatures, and theory for

electronic transition might play a role in this process.

It has been largely accepted that Titan could represent a model of primitive earth (Raulin et al., 2009).

The chemistry of carbocations in the nitrogen-methane dominated ionospheres of Titan is immensely

important, and this has a possible bearing on organic evolution in the atmosphere of early earth and

likely provides the first steps toward a mechanistic theory of aromaticity in terrestrial prebiotic

18

chemistry. The radio and infrared observations in the atmospheres of extrasolar planets and in Titan’s

atmosphere as well should be sought to place a link between organic atmospheric chemistry on Titan

and the chemistry in the atmosphere of young earth at its earliest stage of formation in our solar

system.

•Formation of terrestrial aromatic cyclic compounds in space is a long standing problem . The long

debated Olah’s carbocationic chemistry (Olah, 1995; and references therein) has been accepted as a rule

rather than exception in physical organic chemistry. In this paper we have identified that an

understanding of molecular structures and reactivity of carbocations (detected by Cassini instruments)

and their relevance to “nonclassical” carbo-ions will explore aromaticity in the atmosphere of Titan and

chemical pathways toward prebiological molecules. Would the chemistry of carbocations be observable

in the atmosphere of extrasolar planets? Such astrophysical observational search will most likely place a

link between abiotic organic synthetic chemistry in Titan’s atmosphere and terrestrial prebiotic

chemistry and the origin of life on earth.

The key argument presented in this paper rests on thermochemistry, chemical kinetics and dynamics for

the Huckel aromatic structure determination of INMS detected composition (Waite et al., 2004, 2009;

Vuitton et al., 2009a) of specific carbocations. To summarize, we have identified an important class of

ion-molecule reactions proceeding via carbocation collision complexes, and its implications in chemistry

in Titan’s upper atmosphere. Based on the kinetics and dynamics of the elementary reactions identified,

an analysis of the compositions of ions detected (Vuitton et al., 2009a; Waite et al., 2009) by Cassini

cationic mass spectrometry (Waite et al., 2004) reveals the mechanism of formation of the three-

membered Huckel aromatic rings -the cyclopropenyl cation and its cyclic methyl derivatives. Emphasis

has been given on a synergic effort combining state-of-the-art laboratory experiments, quantum

theoretical calculations, and astronomical observations (also including planetary in situ measurements)

to elucidate the structure, energetics and dynamics of the compositions of carbocations detected in

Titan’s thermosphere and ionosphere.

Acknowledgements

This work was supported in part at NASA Goddard Space Flight Center by the Cassini Plasma

Spectrometer (CAPS) Project through NASA Jet Propulsion Laboratory contract 1243218 with the

Southwest Research Institute in San Antonio, Texas. C.P. gratefully acknowledges support from Italian

MIUR: PRIN 2009 funds (Project: “Molecular Spectroscopy for Atmospherical and Astrochemical

Research: Experiment, Theory and Applications”). A.A. thanks Dr. J. Hunter Waite for a critical reading of

the manuscript and helpful discussions about the limitations of INMS measurements. We are grateful to

both reviewers for a careful reading of the manuscript and for comments. One of the referees

(anonymous) summarized the version of our paper . We appreciated his summary so much that we have

decided to paraphrase his abstract, and this is included in the last section of the manuscript in a bulleted

format.

19

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FIG.1. Schematic potential energy surface for the CH3+ + C2H2 system. The delocalized Allyl cation is the

global minimum. Both products cyclic-C3H3+ and linear-C3H3

+ are shown along with the isomerization

barrier separating them. The larger circles in structures denote carbon atoms. Three collision complexes

and nine transition states are schematically shown in the reaction coordinate plot. The numbers denote

zero-point corrected energies (in Kcal/mol) relative to the reactants.

FIG.2. Calculated pure rotational spectra for 13C and deuterium-containing isotopologues of cyclic and

linear C3H3+ in the 20-150 GHz frequency range.